Electrical woze

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Willy
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Electrical woze

Post by Willy »

So, I buy a BX which is superb in every way other than it doesn't work. Slow cranked alot when I got it, then after a few days of not driving it didn't start at all (cranked slower and slower then fizzled out). Jump started it and it worked fine, yay. Put in a new battery, it started fine, again yay. Tested it and the alternator was putting across a healthy 14V so ruled that out as a fault. However, after not driving for six days, tested the battery and it's now gone down to 11.7V, it's slow cranking and not starting. Back to square one.

What's happened/likely remedy?

:(
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Re: Electrical woze

Post by Tinkley »

Personal experience of draining battery has been leak through under washer bottle n/s shorting fuse relays. Take out the washer and check. Only other thing causing slow cranking for me was worn starter Brushes but it was almost the reverse symptoms. It started slow and then got faster. If it went down during the rain last week its definitely worth checking the fusebox. Drop it down on the hinges and gently peel back the black rear cover underneath it. If there is water there it will seep. Also the carpet in the footwell will be damp.

Only my limited experience and not on diesel. It is worth checking the main battery ground is clean and good. On the petrols its to the rear of the gearbox, similar to the type of gearbox you have.
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Re: Electrical woze

Post by sdelasal »

willy - sounds like you've got excess current drain from the battery - is there a light on somewhere for the 6 days? Try re-charging the battery, test the engine cranks - then disconnect for 6 days, connect it back up & see if it cranks. Simpler would be to measure current drain from the battery. If it's not current drain, then your new batter is no good - take it back & ask them do test it.
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electrokid
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Re: Electrical woze

Post by electrokid »

It does sound as though there is some current being drawn all the time. Sometimes the electronics in the alternator can get a bit leaky: also check the lamp in the glovebox to make sure it goes out when you shut the lid.
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Re: Electrical woze

Post by Defender110 »

and check the boot lamp switch is working.
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Willy
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Re: Electrical woze

Post by Willy »

The glovebox light was stuck on

But still, alternator only putting out 13.5V, this a bit too low or within tolerance? My flatmate's Polo put out 14.5 earlier.
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Re: Electrical woze

Post by themildbunch »

Sounds like my symptoms... found an unused but old alternator cheaply - seemed to charge but at a lower rate like yours and when you turn the ignition off the battery light comes on! After 8 hours or so the battery is completely drained....

A dodgy diode in the alternator, I think... I've been unhooking the battery when parked up!
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Re: Electrical woze

Post by Tinkley »

When we test the outboards down the Sailing Club we normally get 14 to 14.5v from the alternator. We had a voltage regulator/rectifier unit fail and this drained the battery on one engine. I've had an alternator failure on a BX, a slow gradual thing. Try a sharp not brutal whack with a hammer, on the alternator, sometimes it dislodges dirt inside and it will carry on working OK. Exchange is around 50-60 quid for a new one but you might find a bargain. You need to isolate the problem first though.
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Willy
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Re: Electrical woze

Post by Willy »

An hour after my spin, tested the battery. Now on 11.75V. Is this too low for a battery or will it "naturally" settle down to that lower voltage?

Ran the engine again, alternator feeding 13.6V across the battery terminals (am I testing this correctly or should I use a ground point?).

Engine off, battery on 12.2V, then started going down slowly. :?
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Willy
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Re: Electrical woze

Post by Willy »

Okay so update, turns out it's putting out 13.5V across the terminals with nothing on, put on some lights and that it's only 12.9V :???:?
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Re: Electrical woze

Post by Thread Bear »

I have had a radiator fan fail and remain on, draining current, on a Pug 405. All down to corrosion. BX is much the same system so a rad fan check as per that thread? A badly fitted alarm even if it is 'not in use' can have the same effect. Same Pug after it was struck by lightening. The alarm was very dodgy ever after and best left off.
Both those faults were found by pulling the fuses and disconnecting the live circuits not fused and working through them with a tester one by one. Neither are straight forward circuits. The fans have several relays and switches and it needs two failures really to lean on the battery, so it was not immediately obvious where the problem was.
No experience of a naff alternator. Mine just packed up. But they rely on diodes so I guess one might leak and it still charge. Normally you know they are going as the headlights change their brightness with revs, and the cut off for charging if the Battery is good. Also a sign of a crappy earth.
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Re: Electrical woze

Post by mat_fenwick »

Alternator voltage is too low especially under load. Battery voltage isn't healthy either, as you should be looking at ~12.5 V when charged. But this may just be down to the alternator not charging it properly, as the battery is new therefore should be OK! Do you have a mains charger you can use? It's not good for lead acid batteries to leave them discharged for long.

Is there any spark when disconnecting the battery terminals (with everything turned off and doors shut)? Dunno what current capabilities your meter has so the spark test checks for large currents before risking damage to the meter.

Next step would be to check (with a meter) the wiring from the alternator in case the voltage drop when lights on is occurring over the wiring, but I doubt it.
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Re: Electrical woze

Post by KevR »

Tinkley wrote:Try a sharp not brutal whack with a hammer, on the alternator, sometimes it dislodges dirt inside and it will carry on working OK. Exchange is around 50-60 quid for a new one but you might find a bargain.
Had this problem on the grey one last year - whacking it worked a treat for several months, then it packed up properly. It's because the brushes and slip ring are worn, and the brushes are straining at the very end of their springs so whacking it allows the brushes to jiggle down just a tiny bit further, and maybe dislodges some crud. I'd recommend whacking it as a get you home measure, then take it out and replace the brushes (which cost a couple of quid). That way you can get a load more life out of the slip rings without burning them out (when I stripped my alternator the lower of the two slip rings was worn/burnt pretty much right through). Slip rings themselves are only a few quid, but a bit trickier to change.
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Re: Electrical woze

Post by electrokid »

Measuring voltage across the battery terminals is fine - but do check that the earth strap from battery / bodywork / engine is solid as a poor connection there will screw things up quite nicely.

A battery off load and "flat" will measure around 11.6 volts. When fully charged they can be anywhere between 13 and 14 volts - chemistry has changed slightly over the years so differences at full charge can be expected.

If you have a "normal" or "simple" mains battery charger you can give the battery a reasonable check - after it's been on charge for quite a while and the current has fallen to a low value then the battery voltage should be around 14 volts (while still on charge). Voltages higher than that normally indicate a battery that has a higher than normal internal resistance - it's getting old and knackered (like me :roll: ).

When switching off the charger it's normal for the battery volts to drop slowly and settle to anything between 12.5 and 14 volts - typically 13 ish. Measure the voltage a half hour after the charger has been disconnected and then without any kind of load (preferably disconnected from the car) measure again the following day - there should be very little difference between the 2 readings - if there is then there may be excessive "self discharge".

The alternator is a 3 phase AC machine - rectified using diodes to produce a DC charge. Sometimes diodes blow and the sort of symptoms of low charge volts you are describing are similar to what happens when 2 of the phases are duff and only one phase remains working - but check that earth strap before jumping to the expensive conclusion re the alternator. Other faults are available - if the charge volts changes strangely in relation to engine revs then an eccentric slipring may be the cause - typically charge will be ok at say 1500 rpm but reduce as the revs increase and the brushes get thrown off the slipring.
Is there any spark when disconnecting the battery terminals (with everything turned off and doors shut)?
Very good test - and one we often forget to try !

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Re: Electrical woze

Post by Jaba »

I have several alternators surplus here which I am going to throw out Will if you decide you want to replace yours.

P.S. I haven't checked my headlamp stocks yet.
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